The laws of optics teach that the image of a point object which is not situated on the optical axis of a given objective is also not situated on the optical axis. Thus, when a point object moves along an axis parallel to the optical axis of the objective, the new position of the image point is deduced from that preceding by the combination of a translation in parallel to the optical axis and of a translation perpendicularly to this axis.
This leads to the fact that, when at least two objectives each form a point image of one and the same point object, and when this point object moves parallel to the optical axes of these objectives and along the general axis of symmetry of the system, the point images obtained move, on the one hand, in parallel to the optical axes and, on the other hand, perpendicularly to these axes in opposite directions.
Consequently, a single image from the images produced individually by each objective, by superposing these images, can only be obtained for points which are situated at infinity. The individual images of other points cannot be superposed which results in a fault called parallax.
It is known nowadays to reduce this parallax error by slightly inclining the optical axes of the various objectives in relation to the general axis of the system, when focusing.
However, this solution results in adjustments which are always highly sensitive and imperfect, since there remains a fault consisting of a trapezoidal distortion of the image produced by each objective, this fault producing a spatial distortion which is impossible to correct when the individual images are superposed to produce a single image.